The electronic line switch fulfils a number of functions including a hook-switch function, that is, the line switch acts as a hook-switch when a hook-switch control signal is selectively applied to the electronic line switch, a hook-switch control signal being generated when the user brings the subset into the off-hook mode. This control signal may, for example, be provided by a ten number repetory tone/pulse dialler chip incorporating a CMOS push-pull dial pulse output.
In a subset provided with a conventional mechanical hook-switch and dialler chip, the dialler chip is provided with its operating power from current derived from the exchange battery and drawn over the exchange line through the operated hook-switch. As soon as the subset is brought into the off-hook mode the hook-switch contact operates and sufficient operating voltage is immediately extended to the dialler chip which can then function.
In the case of a subset provided with an electronic line switch, however, when such a subset is initially connected to an exchange line, or re-connected after subsequently being un-plugged, the dialler chip would be without power because its power source is cut off by the very line switch it controls. Consequently the dialler chip would be unable to function and the line switch could not be signalled. Therefore it is necessary in such a case to provide control circuitry to ensure that on the one hand at least a mimumun operating voltage be provided for the dialler chip when the subset is in the on-hook mode, and on the other hand the line switch remains off in the on-hook mode and that the only current drawn from the exchange line is a small bleed current which does not exceed a predetermined value, typically 50 .mu.A.
In the specification of Australian Patent No. 606397 (which corresponds to International (PCT) application PCT/AU88/00046), which was published by WIPO under publication number WO 88 08 234 and which is hereby incorporated by reference, there is described a circuit for providing initial power to a dialler chip associated with a telephone subset having an electronic line switch of the aforementioned type. This known circuit comprises a controllable semiconductor switch controlled by a control circuit, which switches the electronic line switch on when the control circuit senses that the voltage on the dialler chip's power terminals is below a predetermined voltage, and allows a storage capacitor coupled across the power terminals to be charged by line current flowing via the operated line switch. The controllable semiconductor switch switches the line switch off when the voltage across the capacitor rises to a predetermined magnitude sufficient to allow the dialler chip to provide hook-switch signals to the line switch.
A disadvantage of the aforementioned known circuit is that when the subset is connected in parallel with a low voltage modem the data transmission from the modem is corrupted by noise generated in the control circuit of the subset which charges the storage capacitor coupled across the dialler chip's power terminals. Another disadvantage of the aforementioned known circuit is that the number of circuit elements therein make it relatively expensive.
It is an object of the present invention to provide a telephone subset line switch control circuit which ensures that the line switch remains off when the subset is in the on-hook mode, and current from the exchange line is coupled via a bleed circuit means to maintain the subset's dialler chip's normal operating voltage.
It is a further object of the present invention to provide a telephone subset line switch control circuit which ensures that when the voltage across the dialler chip's power terminals is below the minimum operating voltage, the operation of the hook-switch to take the subset into the off-hook mode causes line current to be diverted into the line switch's control element so that the line switch allows sufficient current to be drawn therethrough to provide minimum operating voltage to be applied to the power terminals of the dialler chip.
It is a further object of the present invention to provide a telephone subset line switch control circuit which will turn the line switch on fully and quickly when the subset is brought off-hook, except during hook-switch flash and dialling break periods whereupon the line switch is fully off.
It is a further object of the present invention to provide a telephone subset line switch control circuit which includes means to maintain the dialler chip's memory for at least a predetermined minimum period after disconnection from the exchange line.
It is a further object of the present invention to provide a telephone subset line switch control circuit which does not corrupt data transmitted by a device connected in parallel with the said subset.
It is a further object of the present invention to prevent current leakage within the line switch control circuit from activating the line when the subset is in the on-hook mode.
It is a further object of the present invention to provide a telephone subset line switch control circuit which comprises relatively few circuit components.
According to the present invention, in a telephone subset comprising a first and a second line terminal means for respectively connecting to conductors of an exchange line; a speech circuit means connected between the line terminal means; a line switch means whose switching element is formed by a conductive path of a first controllable semiconductor switch means having a control element coupled to a line switch control terminal means, said switching element being serially connected in the said subset's loop current circuit; and a dialler circuit means having power supply terminal means, a hook-switch terminal means coupled to hook-switch contact means, and a signal output terminal means, a method for controlling said first controllable semiconductor switch means comprising the steps of:
(a) deriving current from the exchange line with a current bleed circuit means to maintain a first predetermined voltage across said power supply terminals when said hook-switch is in the on-hook mode to partially energize said dialler circuit means. PA1 (b) detecting an off-hook condition of said hook-switch with a second controllable switch means which is rendered conducting thereby. PA1 (c) directing current from said exchange line into said line switch control terminal means via said second controllable semiconductor switch means to allow sufficient current to flow through the said conductive path of said first controllable semiconductor switch means to charge a first storage capacitor means to a second predetermined voltage for fully energizing said dialler circuit means, whereupon said first controllable semiconductor switch means is saturated and more current flows through said conductive path of the first controllable semiconductor switch means whereby said first storage capacitor means is charged to a third predetermined voltage. PA1 (d) providing a third controllable semiconductor switch means arranged to be rendered conducting upon detection of said third predetermined voltage together with detection of a break signal condition on said signal output terminal means of said dialler circuit means, thereby diverting current from said exchange line into said signal output terminal means and rendering said first controllable semiconductor switch means non-conducting thereby breaking the said subset's loop current circuit. PA1 (e) provide a second storage capacitor means to maintain a voltage equivalent to said first predetermined voltage across said power supply terminals for a predetermined minimum period when said line terminal means are subsequently disconnected from said conductors of the exchange line.
According to a further aspect of the present invention there is provided a telephone subset comprising a first and a second line terminal means for respectively connecting to conductors of an exchange line; a speech circuit means connected between the line terminal means; a line switch means whose switching element is formed by a conductive path of a first controllable semiconductor switch means having a control element coupled to a line switch control terminal means, said switching element being serially connected in the said subset's loop current circuit; and a dialler circuit means having power supply terminal means, a hook-switch terminal means coupled to hook-switch contact means, and a signal output terminal means, wherein said subset further includes a current bleed circuit means for deriving current from the exchange line to maintain a first predetermined voltage across said power supply terminals when said hook-switch is in the on-hook mode to partially energize said dialler circuit means; a second controllable switch means which is rendered conducting upon detecting an off-hook condition and directing current from said exchange line into said line switch control terminal means via said second controllable semiconductor switch means to allow sufficient current to flow through the said conductive path of said first controllable semiconductor switch means to charge a first storage capacitor means to a second predetermined voltage for fully energizing said dialler circuit means, whereupon said first controllable semiconductor switch means is saturated and more current flows through said conductive path of the first controllable semiconductor switch means whereby said first storage capacitor means is charged to a third predetermined voltage; a third controllable semiconductor switch means arranged to be rendered conducting upon detection of said third predetermined voltage together with detection of a break signal condition on said signal output terminal means of said dialler circuit means, thereby diverting current from said exchange line into said signal output terminal means and rendering said first controllable semiconductor switch means non-conducting thereby breaking the said subset's loop current circuit; a second storage capacitor means maintaining a voltage equivalent to said first predetermined voltage across said power supply terminals for a predetermined minimum period when said line terminal means are subsequently disconnected from said conductors of the exchange line.
Preferably, the second controllable switch means is a transistor controlled by the hook-switch contacts, although this arrangement could be substituted by an additional set of hook-switch contacts.